Two-component, primeness polyurethane structural adhesives have been used for many years in industry to adhere substrates such as fiberglass reinforced plastics (FRP). These materials are also commonly referred to as sheet or bulk molding compound (SMC, BMC).
U.S. Pat. Nos. 4,923,756 and 5,002,806 relate to two-component primeness polyurethane adhesives used to bond FRP and SMC. Urethane adhesives are preferred to other adhesives because of outstanding characteristics such as being primerless, non-sagging on vertical surfaces, optional long open-times, flexibility, shock and impact resistance, and high bond strengths.
Typically, two-component polyurethane adhesives are prepared by combining a polyisocyanate prepolymer with a curative component just before applying the mixture to a first substrate. A second substrate is placed over the adhesive and then the laminated part is cured at 300.degree. F. (149.degree. C.) for 30 minutes.
Since adhesives may be applied to substrates in a vertical position, sag resistance is preferred. This property is known as "non-sag" and is measured as the distance the adhesive flows during a certain time period; such as three minutes. ASTM D2202-84 is the test method used to measure sag.
It is important that the adhesive remain partially uncured and fluid for a sufficient amount of time to permit placing the second substrate on to the adhesive and acquiring sufficient wet-out necessary for good adhesion. The length of time the adhesive remains fluid is referred to as the "open time".
In addition to being primerless, other advances in polyurethane adhesives include enhanced adhesion and reduced moisture sensitivity. This is done by including specific additional compounds. U.S. Pat. Nos. 5,175,228 and 5,340,901 relate to two-component, primeness polyurethane adhesives which contain phosphorus-type adhesion promoters. In both references, the urethane prepolymer component contains a large excess of free isocyanate, while the curative component contains a polyol curing agent, an amine sag resistant agent, and an organic phosphorous compound.
Conventional urethane adhesives are known to have foaming problems, especially when bonding in high humidity environments or when the substrate surface is contaminated with moisture. This often results in weaker bonds. U.S. Pat. No. 4,728,710 relates to an improved anti-foaming two-component polyurethane adhesive containing a polyisocyanate prepolymer component and a curative component composed of a polyol, a polyamine, and a bicyclic amide acetal. Foaming at the adhesive / substrate interface, which was probably caused by the reaction of isocyanates with moisture, was reduced or eliminated due to the fast reaction between bicyclic amide acetal compounds and moisture.
European Patent 0,149,765, Canadian Patent 2,120,860 A1 and U.S. Pat. Nos. 3,932,357, 4,108,842, 5,288,797 and 5,412,056 relate to one and two-component urethane coatings and films comprising a polyisocyanate prepolymer component and optionally a curative component composed of ketimines/aldimines and optionally amines. The imine compounds react with moisture in the air forming carbonyl compounds (typically low molecular weight volatile organic compounds) and amine compounds which subsequently react with isocyanates to cure the coating or film. Moisture is usually required to cure, while foaming is eliminated due to the favorable reaction between imines and water.
Ketimines and aldimines of specific structures are also known to react with organic isocyanates as described in Polymer Materials Science and Engineering, vol. 55, pp. 448-457, 1986.
Polyisocyanate and polyurethane components cured with imine compounds have been used as two-component adhesives. German Patent 2,630,013 A1 discloses the use of a polyimine combined with a polyester urethane component for bonding plasticizer-containing polyvinyl chloride. German Patent 4,006,537 A1 relates to a two-component adhesive which is composed of a polyisocyanate prepolymer component and a curative component containing primarily an imine compound. The curative contains a minimum 40% imine-reactive groups, optionally, less favorable compounds such as polyols and polyamines were added up to a maximum of 60% reactive groups. U.S. Pat. No. 4,554,299 relates to an imine used as a curative of an isocyanate/epoxy three-component system.
It would be an improvement in technology if two-component urethane adhesives were primeness and demonstrated consistent enhanced adhesion when bonded in humid as well as dry environments. Imine compounds have been used to eliminate foaming in moisture-cured coatings and to cure isocyanate compounds, but no one has described a curative containing less than 40 weight % of an imine compound used to enhance adhesion and increase moisture resistance of a polyurethane adhesive.